This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. xc2xa7119 from an application entitled METHOD FOR ESTABLISHING SEARCH WINDOW SIZE OF A MOBILE STATION IN A CELLULAR SYSTEM earlier filed in the Korean Industrial Property Office on June 25, 1998, and there duly assigned Serial No. 1998-24069.
1. Field of the Invention
The present invention relates to a method of establishing a search window size of a mobile station in a cellular system, more particularly, a method for estimating the search window size wherein the mobile station searches for a pilot signal transmitted from a base transceiver station (BTS) in the cellular system.
2. Description of the Related Art
A cellular mobile telecommunication system divides the total service coverage into a plurality of radio BTS areas. A BTS area, also called a cell, has a small service coverage area. Several BTS""s may be controlled centrally by a mobile switching center (MSC), which makes it possible for subscribers to continue to communicate with each other while moving between cells.
FIG. 1 illustrates a design configuration of a cellular system using code division multiple access (CDMA) technology. As illustrated in FIG. 1, the cellular system includes a mobile station (MS) 40, which is provided with mobile telecommunication services; base transceiver stations (BTS) 30, 31 which provide the mobile station 40 with services; base station controller (BSC) 20 controlling the base transceiver stations 30, 31; and mobile switching center (MSC) 10 connecting the base station controller 20 to the general public switched telephone network (PSTN) 11. The mobile switching center 10 provides services by obtaining the information about the mobile station 40 from home location register (HLR) 12 and visitor location register (VLR) 13.
In the CDMA system configured as above, a link from the base transceiver station to the mobile station is called a forward link and a link from the mobile station to the base transceiver station is called a reverse link. All forward links in a base transceiver station have the same pseudo-random noise (PN) sequence time offset. The PN offset is transmitted via a pilot channel, called a pilot signal, as one of the forward channels. The pilot signal is an identification signal which identifies the plurality of base transceiver stations in the CDMA system.
The CDMA mobile station always monitors the pilot signal in order to obtain a good quality signal on the forward channel. The mobile station located within the service coverage of the base transceiver station uses the pilot signal for synchronization. The mobile station can acquire a timing of the forward CDMA channel from the pilot signal and obtain the phase reference for coherent demodulation.
The CDMA mobile station can simultaneously communicate with a plurality of base transceiver stations while differentiating the code. The above function describes a soft handoff, where a searcher is used for selecting the best quality path among the plurality of base transceiver station""s paths. That is, each of the base transceiver stations transmits a pilot signal having its own PN offset, so the mobile station searches for the timing location from which the strongest pilot signals are received. This step is called pilot signal acquisition.
After the mobile station receives the pilot signal from the base transceiver station, the base transceiver station continues to instruct the mobile station to search for transmitted pilot signals from other base transceiver stations as well as using a particular timing location for each base transceiver station. The CDMA system performs transmission diversity schemes using multiple digital receivers to reflect signals transmitted from a base transceiver station through the multipath. Such multiple digital receivers are called a fingers.
At this time, the mobile station has the timing interval to search for the pilot signals and the base transceiver station provides the mobile station with a particular timing interval for each pilot signal, i.e., window size via a paging channel. When the mobile station is initially connected to the base transceiver station, the base transceiver station transmits the search window corresponding to each set per active set/neighbor set/candidate set/remain set, to the mobile station. The search window transmitted from the base transceiver station is fixed for the corresponding base transceiver station.
The searcher of the mobile station detects the strongest pilot signal, i.e., PN offset, continuously within the search window among the currently connected plurality of pilot signals. If the search window is too large, it takes too much time to search for the strongest pilot signal, and if it is too small, it is difficult to get the exact search result. Therefore, it is very important to establish the appropriate search window size in order to obtain a good quality signal on the forward channel.
Two methods of establishing the search window size according to the prior art is a modeling method and a measuring method. The modeling method estimates the search window size through the multipath model. However, the search window is variable according to the multipath model which creates a practical limitation based on the field environment.
The measuring method measures the search window in such a way that a receiver measures the delay spread through a power profile for the outputted continuous wave. This method has more reliability than the modeling method. However, a disadvantage is that the method does not consider the characteristic of the mobile station, itself.
U.S. Patent No. 5,726,982, entitled xe2x80x9cArtificial Window Size Interrupt Reduction System For CDMA Receiverxe2x80x9d discloses a method for acquiring phase information of a PN sequence via the mobile station in the CDMA system by previously establishing the window size. The method establishes the search window center per active/candidate/neighbor/remain set and establishes the search window by adding an upper/lower limit which is provided from the base station to the search window center. However, the disadvantage remains in that the method does not consider the characteristic of the mobile station or the characteristic of the practical radio morphology.
To solve the disadvantages of the prior art as stated above, the present invention provides a method of establishing a search window size for a mobile station in a cellular system having a present radio morphology. One preferred embodiment of the method according to the present invention, wherein the mobile station is provided with communication services from a plurality of base transceiver stations (BTSs), and each of the plurality of BTSs has an inherent pilot signal, and the mobile station searches for the pilot signal at each searcher position having a constant time interval within a search window on a time-axis, includes: finding a correlation energy value at each searcher position by which a searcher of the mobile station searches for pilot signals received from the plurality of BTSs at each searcher position within a predetermined first search window and outputting a result of the searching; gathering the result of the searching outputted from the searcher of the mobile station using a diagnostic monitor (DM) connected to the mobile station; and estimating the size of a second search window to be applied to the present radio morphology based on the gathered result of the searching and applying the second search window size to the mobile station.
Another preferred embodiment of the method according to the present invention, wherein the mobile station is provided with communication services from a plurality of base transceiver stations (BTSs), and each of the plurality of BTSs has inherent pilot signal, and the mobile station searches for the pilot signal at each searcher position having a constant time interval within a search window on a time-axis, includes: finding a correlation energy value at each searcher position by which a searcher of the mobile station searches for pilot signals received from the plurality of BTSs at each searcher position within a predetermined first search window; gathering maximum index samples from the searcher of the mobile station by a diagnostic monitor (DM) connected to the mobile station, wherein the maximum index samples include information denoting the location of a searcher position from a center of the first search window, wherein the searcher position has the highest correlation energy value within the first search window; establishing confidence level value, p, for the gathered maximum index samples; calculating a mean value, m, and a standard deviation, "sgr", for the gathered maximum index samples; calculating a confidence interval for the gathered maximum index samples using the established confidence level value, p, the calculated mean value, m, and standard deviation, "sgr", and estimating the size of a second search window to be applied to the present radio morphology using the calculated confidence interval and applying the estimated second search window size to the mobile station.